1. Field of the Invention
The invention relates generally to sense amplifiers, and more particularly to circuitry that improves input slewing on sense amplifiers having relatively high or unpredictable capacitive inputs.
2. Description of Related Art
The trends in electronic circuit design are increases in speed, levels of integration, and the seemingly inconsistent--portable operation. This is so since increases in speed and integration typically increase power consumption which is detrimental to battery operation. However at the expense of power consumption, power hungry sense amplifiers have been used in a number of circuit applications to speed-up the transition time between changes in logic levels. For the most part, this is accomplished by compressing input levels to just above or below-the switch point of a sensing inverter. The swing or "slew" of the input therefore, need not be as large to trigger states. This level "compression" however is not without limitations--including the increase in power consumption.
By way of background, FIG. 1 depicts a typical prior art sense amplifier 10 having an input node 14 coupled to receive logic input signals from an N-channel MOSFET logic tree 12. The input node 14 is coupled to the sources of load P-channel transistor 28 and fast recovery N-channel transistor 36, and to an input on sensing inverter 32. An output on inverter 32 drives the gate of fast recovery transistor 36, an input on feedback inverter 30, and an input on output driver inverter 34. An output on feedback inverter 30 drives the gate of load transistor 28. The drains of load transistor 28 and fast recovery transistor 36 are coupled to the supply voltage V.sub.DD . The output voltage on driver inverter 34 swings substantially between ground and V.sub.DD to restore logic levels to their normal amplitudes.
In an improved but not entirely satisfactory technique, sense amplifiers have been constructed with enable lines to enable power consuming activities only when necessary. Another limitation however, with most sense amplifiers in general, including those with enable lines, is the inability to swiftly pull the input node 14 to V.sub.DD (i.e. recover to a high state) when the input node 14 dynamically exhibits a high or unpredictable capacitive impedance. This condition typically occurs when either: (i) the sense amplifier has an enable line which is dynamically enabled, (ii) the logic tree 12 dynamically presents multiple pull down paths, or (iii) the logic tree 12 is dynamically changed such as in a read only memory (ROM) row/column select mechanism, described in more detail hereinbelow.
As an illustrative example, reference is now made to FIG. 2 which depicts a typical ROM row/column select mechanism 40 employing a sense amplifier 10. The input capacitance on input node 14 to sense amplifier 10 is highly dependent and dynamic upon which row is selected by row select circuitry 42 and by which column is selected by column select circuitry 44. It is conceivable therefore, that the input capacitance on input node 14 to sense amplifier 10 will radically and dynamically change if the column select circuitry 44 selects a column before the row select circuitry 42 has properly selected a row. More specifically, it is possible that the input node 14 will see "N" transitory row selections pulling to ground if the row select circuitry 42 is undefined when the column has been selected. Accordingly, the slew rate of sense amplifier 10 is slowed when the quiescent row selection is high and "N" transitory row selections are pulling to ground (or exhibit a high capacitance) since transistors 28 and 36 in the sense amplifier 10 haven't had a chance to recover (i.e. charge the input capacitance).
The typical approach in addressing file aforementioned problem is to add individual sense amplifiers for each column select line to ameliorate the effects of row/column select loading. Adding such sense amplifiers however, not only dramatically increases power consumption, but also increases component count, conditions which are extremely unfavorable in portable environments.
It can be seen therefore, that there is a need to increase the slew rate of a sense amplifier in situations of large or unpredictable input capacitance, without the need to add more sense amplifiers.